Basically, the conventional pixel circuit in an organic light emitting diode (OLED) display apparatus is mainly implemented by two transistors and one capacitor which are used for corporately controlling the brightness of the organic light emitting diode. However, the circuit design of the conventional pixel circuit may result in a non-uniformity issue.
FIG. 1 is a schematic circuit view of a conventional pixel circuit. As shown, the conventional pixel circuit 100 mainly includes two transistors 101 and 102, a capacitor 103 and an organic light emitting diode 110. Each one of the transistors 101 and 102 has a first terminal, a second terminal and a control terminal; and the capacitor 103 has a first terminal and a second terminal. Specifically, the transistor 101 is configured to have the first terminal thereof directly connected to a power voltage OVDD. The transistor 102 is configured to have the first terminal thereof for receiving display data DATA, the second terminal thereof electrically coupled to the control terminal of the transistor 101, and the control terminal thereof for receiving a scan signal SCAN. The capacitor 103 is configured to have the first terminal thereof directly connected to the first terminal of the transistor 101 as well as the power voltage OVDD and the second terminal thereof directly connected to the second terminal of the transistor 102 as well as the control terminal of the transistor 101. The organic light emitting diode 110 is configured to have the anode terminal thereof electrically coupled to the second terminal of the transistor 101 and the cathode terminal thereof directly connected to a power voltage OVSS. Through the aforementioned circuit configuration, the pixel circuit 100 may control the current flowing through the organic light emitting diode 110 according to the cross voltage between the control terminal of the transistor 101 (i.e., the connecting node G) and the second terminal of the transistor 101 (i.e., the connecting node S); wherein the current flowing through the organic light emitting diode 110 is obtained by the following equation:IOLED=K*(VGS−|VTH|)2 
where IOLED is the current flowing through the organic light emitting diode 110; K is a constant; VGS is a voltage difference between the connecting nodes G and S which are related to the power voltage OVDD and the display data DATA, respectively; and VTH is the threshold voltage of the transistor 101.
However, because each one of the pixel circuits 100 is electrically coupled to the power voltage OVDD through the respective metal line and each metal line may have an impedance which may lead to an IR-drop, the pixel circuits 100 may receive different power voltages OVDD and have different pixel currents IOLED flowing therein, and consequentially the pixel circuits 100 may have different brightness and thereby resulting in the non-uniformity issue. In addition, because the pixel transistors 101 in the respective pixel circuits 100 may have different threshold voltages VTH due to the different manufacturing processes, the pixel circuits 100 may have different pixel currents IOLED flowing therein, and consequentially the pixel circuits 100 may have different brightness and thereby resulting in the non-uniformity issue.
In addition, because the organic light emitting diode 110 may have an increasing resistance with the operation time and the material decay, the second terminal of the transistor 101 (i.e., the connecting node S) may have an increasing voltage and consequentially the transistor 101 may have a decreasing cross voltage VGS while the organic light emitting diode 110 has an increasing cross voltage. Thus, when the cross voltage VGS decreases and the current flowing through the transistor 101 correspondingly decreases, a decreasing IOLED is resulted in and consequentially the pixel circuits 100 may have decreasing brightness and thereby resulting in the non-uniformity issue.